An enzyme-linked immunosorbent assay was used to examine randomly selected sera from 389 children and adults in Lima, Peru, and 84 children in Maracaibo and Caracas, Venezuela, for IgM or IgG antibody to Cryptosporidium. In Peru and Venezuela, 19.8% and 15.5% of the study populations, respectively, were positive for both specific IgG and IgM antibodies, a result consistent with active or recent infection and representing a larger percentage than normally reported from stool examinations of individuals seeking medical attention. Sixty-four percent of subjects from each country had detectable levels of specific IgG, indicating infection sometime in life. Detection of specific IgG increased in the two-to three-year-old age-group, a result suggesting that this is a common age for infection. Persistence of IgG and, less often, IgM antibody response over 12 mo occurred in some subjects, although the signif cance of this finding is uncertain. Our findings suggest that Cryptosporidium infections are endemic in the communities surveyed and that most residents have been infected. © The University of Chicago.